1. Field of the Invention
The present invention relates to a fuel supply apparatus for an internal combustion engine, and particularly to a fuel supply apparatus for injecting fuel directly into a combustion chamber of the internal combustion engine.
2. Description of the Related Art
Japanese Patent Laid-open (Kokoku) No. H07-72511 discloses a fuel supply apparatus in which fuel pressurized by a fuel pump is retained in a fuel accumulator (common-rail), and the pressurized fuel is supplied from the fuel accumulator to a fuel injection valve. A technique for rapidly converging a fuel pressure in the fuel accumulator to a target fuel pressure is disclosed. According to this apparatus, a discharge amount of the fuel pump is controlled to the minimum and an electromagnetic valve provided in the fuel accumulator is opened, to rapidly converge the fuel pressure in the fuel accumulator to the target fuel pressure, when the fuel pressure in the fuel accumulator exceeds the target fuel pressure by a predetermined value.
There is a problem described below in a case where a common-rail, which is provided with a relief valve which opens when the fuel pressure becomes abnormally high, is used and it takes a long time period for the relief valve to return to the initial closed state once the relief valve opens.
The fuel pressure in the common-rail temporarily drops if air mixes in the fuel in a passage from the fuel tank to the common-rail. Consequently, a feedback control for increasing the discharge amount of the fuel pump is performed when the fuel pressure in the common-rail drops. In such state, if the mixed air vanishes, the discharge amount of the fuel pump becomes excessive. Accordingly, the fuel pressure in the common-rail abnormally rises, to open the relief valve. Consequently, operating performance of the engine deteriorates in the period until the relief valve returns to the initial closed state. In the technique disclosed in Japanese Patent Laid-open (Kokoku) No. H07-72511, the electromagnetic valve provided in the fuel accumulator corresponds to the relief valve and the actual fuel pressure is rapidly converged to the target fuel pressure by opening the electromagnetic valve. That is, the technique disclosed in Japanese Patent Laid-open (Kokoku) No. H07-72511 does not tend to avoid opening of the electromagnetic valve (relief valve), and hence cannot prevent the above-described inconvenience caused by opening of the relief valve.